IEEE 802.11 standard uses a carrier sense multiple access with collision avoidance (CSMA/CA) mechanism to access a wireless medium (hereinafter medium). A client station (STA) typically listens to the medium (i.e., senses the medium) before transmitting a signal on the medium. There are two methods for sensing the medium. A first method includes physical carrier sensing, which detects presence or absence of RF energy in the medium. A second method includes virtual carrier sensing, which detects presence or absence of an 802.11 signal in the medium.
A Clear Channel assessment (CCA) is a logical function in a physical layer (PHY) of a STA or an access point (AP) that determines a current state of use of the medium. For example, in wireless devices (AP/STA) that use orthogonal frequency division multiplexing (OFDM), a CCA level (threshold) for a 20 MHz channel is typically −82 dBm, and the CCA level (threshold) for a 40 MHz channel is typically −79 dBm. If an energy level detected in a channel is less than the CCA level for the channel, the channel is considered idle, and a transmission in the channel can be attempted. If the energy level detected in the channel is greater than the CCA level for the channel, the channel is considered busy, the wireless device may backoff, and a transmission may not be attempted to avoid collisions.
Another way of minimizing collisions is to assign different color values (typically three bit identifiers in a signal field of a header of a packet) to each basic service set (BSS). Since each BSS has a different color value, a station in a BSS can know after decoding the signal field whether a detected transmission is from within its own BSS to which the station belongs or from a neighboring BSS. Transmissions that are perceived to belong to an overlapping BSS (having a different color value) deferred to by backing off based on a CCA value to avoid collisions.